Files for games installed on Steam are stored in the following folder:

C:\Program Files\Steam\SteamApps\

During the installation of Steam, you have the option to install Steam to a location other than the default. Since Steam relies on the game files residing in the SteamApps folder, your game files cannot be moved outside of this folder. The game files must be in the SteamApps folder in order to function.

If Steam is already installed, you may move the installation to another location, such as a different hard drive using the following process.

Moving Your Steam Installation
Important:

Please ensure that you have your Steam account name and password before following this procedure. Also ensure that your account is linked to your current email address so that you can reset your password if necessary.

It is not recommended that you install Steam to an external hard drive due to potential performance issues.
Warning:

It is highly recommended that you create a backup of your SteamApps folder before attempting this process. If there is a problem in this process and you do not have a backup of your games or the SteamApps folder, it will be necessary to reinstall the games individually. Please see the Using the Steam Backup Feature topic.

The following instructions are a simple way to move your Steam installation along with your games:

Exit the Steam client application.

Browse to the Steam installation folder for the Steam installation you would like to move (C:\Program Files\Steam by default).

Delete all of the files and folders except the SteamApps folder and Steam.exe

Cut and paste the whole Steam folder to the new location, for example: D:\Games\Steam\

Launch Steam and log into your account.

Steam will briefly update and then you will be ready to play. All future game content will be downloaded to the new folder (D:\Games\Steam\SteamApps\ in this example)

If you encounter any errors during the move process or when Steam is launched from the new location, it may be necessary to perform a more thorough process:

Exit the Steam client application.

Browse to the Steam installation folder for the Steam installation (continuing the from the last section example, this would be the new location of D:\Games\Steam.

Move the SteamApps folder from the Steam installation folder to your desktop.

Review the Uninstalling Steam topic for instructions to uninstall Steam (this must be done to remove your old Steam installation settings from the Windows Registry).

Review the Installing Steam topic for instructions to re-install Steam to the desired location.

Move the SteamApps folder into the new Steam installation folder to copy all of your downloaded game content, settings, and saved games to the new location.

Start Steam and log in to your existing Steam account to confirm that the move was successful.

Important:

If you are moving your Steam installation to a new hard drive and want to keep game saves/profiles for third party games that are not saved on Steam Cloud most of these can be found in your Documents folder: ~\Documents[username]\My Games. Moving this folder to the same location on your new hard drive will maintain your saves/profiles.

Steam now allows you to have multiple libraries, so you can have one for each drive. Unfortunately, it doesn't have a built-in way to move a game from one library folder to another (except deleting it and reinstalling it to a different location, which involves downloading it again). But I've been able to move games manually.

If you haven't yet created a library on the destination drive, you can do that by going to Steam > Settings > Downloads > Steam Library Folders > Add Library Folder.

To move a game from one library to another: (I'll assume you want to move "Game of the Year" from C:\Program Files (x86)\Steam to D:\Steam.)

Find out the Steam gameid number for the game you want to move. (You can find this by locating the game's shortcut in the Start menu, right-clicking on it, and selecting Properties. The gameid is the number after steam://rungameid/ in the URL box. Let's say Game of the Year is gameid 1234.)

Exit Steam (make sure its icon is gone from your taskbar)

Find the game you want to move (Game of the Year) in the C:\Program Files (x86)\Steam\SteamApps\common folder. Move it to D:\Steam\SteamApps\common. (Remember that you need to hold down Shift to move files from one drive to another in Windows Explorer. By default, it copies files when crossing drives.)

Find the appmanifest_1234.acf file (where 1234 is the gameid from step 1) in C:\Program Files (x86)\Steam\SteamApps and move it to D:\Steam\SteamApps.

Start Steam. It should automatically realize that "Game of the Year" is now on drive D:.

This doesn't involve symbolic links, so it should work just fine on Windows XP.